what is get_template_directory_uri() equivalent for inside wp-content

I got this code:

function custom_theme_js(){
    wp_register_script( 'infinite_scroll',  get_template_directory_uri() . '/js/jquery.infinitescroll.min.js', array('jquery'),null,true );
    if( ! is_singular() ) {
        wp_enqueue_script('infinite_scroll');
    }
}
add_action('wp_enqueue_scripts', 'custom_theme_js');

what do I need to change get_template_directory_uri() to if I put the above code in an mu-plugin and put the js file in mysite.com/wp-content/js/

Because I need to set this for all my child themes. Basically I don’t know the get_template_directory_uri() equivalent for inside wp-contnent

Thanks in advance